The present invention relates to an injection molding method. More specifically, this invention relates to an injection molding method carried out by dissolving a carbon dioxide gas or a molding method for improving the transcription of products from a die surface or surfaces as well as improvinng a gloss level of the surface or surfaces of molded products.
In injection molding of thermoplastics resin, generally, melted resin is filled in a cavity of a die, and the resin is cooled in the die to obtain molded products having the same form as the cavity. In this injection molding process, as a die is maintained at a lower temperature than the solidifying temperature of the resin. When the melted resin is filled in the cavity of die, freezing of the resin starts and proceeds immediately, and the so-called skin layer, which is a layer subjected to freezing, is formed on the surface of melted resin. Especially, a flow tip of the melted resin is transferred onto a surface of the molded product because of the flow of the melted resin like a fountain, and also in this portion, freezing proceeds and the resin is pressed to the die surface with a low pressure. The results inlude such troubles as transcription defects of the molded product, non-uniformity in flow due to the progress in freezing of the resin, and molding defects arise such as when jetting may occur. To evade the occurrence of such problems as described above, in the conventional technology, a temperature for melting a resin and a temperature of a die are set to higher values to prevent the resin from being solidified and also to improve the transcription.
With this method, however, the transcription is improved, but sink mark and warpage easily occur due to non-uniformity in solidifying and shrinkage of the resin. As such, it is difficult to control the molding process advantageously. In addition, the molding cycle time becomes longer, which results in economical problems such as the higher cost of the molded product.
To solve the problems described above, there has been proposed a method of temporally heating a surface of a die and controling the solidifying of melted resin by making use of a heater, heated oil, heated water, or a heat of the melted resin itself in the injection molding process. With this method, however, the transcription improved, but a specific facility is required, and also the molding cycle time becomes longer, which is economically disadvantageous.
As a method capable of overcoming the problems as described above and also of improving the transcription, Japanese Patent Laid-Open Publication No. HEI 10-128783 proposes an injection molding method. In this injection molding process, before a resin is filled in a cavity of a die, an inactive gas such as a carbon dioxide gas is filled in the cavity and then injected with thermoplastic resin. With this method, however, the resin flows while the inactive gas such as a carbon dioxide gas is dissolved in the resin. This results in flow that is unstable, and sometimes the surface of the molded product may have a defect because of not-uniform flow of the resin and non-uniformity in the flow rate of the resin. Further as a resin is filled in a pressurized cavity space, sometimes insufficient filling may occur in the molded product. It is difficult to control over gas exhausting.
It is an object of the invention to provide an injection molding method that does not cause cosmetic defects and insures an improved transcription and gloss level of products, a reduction of weld lines, and also a reduction of defects such as a flow marks of the resin and a gas marks caused in gas injection molding.
According to the invention, a process of injection molding is provided which improves the surface transcription, gloss level and cosmetic appearance of molded products. The resin is filled into the cavity of a die to form a space between a resin surface and a die surface as to the the visible side of the product(s) to hold down the progress of surface freezing of the melted resin. Into the space fluid is injected which softens the surface of injected resin (skin layer). Subsequently, pressure of the resin in the cavity is increased by adding a holding pressure in order to cohere the resin to the surface of the die. The resin is then cooled down in the cavity of the die until the resin solidifies.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.